On the first 4 exams of his physics class, Brandon got an average score of 79. What does he need on the next exam to have an overall average of 81?
Solution: Let his score on the next exam be $x$ The sum of all of his scores is then $4 \cdot 79 + x$ The same sum must also be equal to $5 \cdot 81$ Solve: $x = 5 \cdot 81 - 4 \cdot 79 = 89$.